The fossilized remains of a previously-unknown species of primate have been unearthed in central China. At 55-million years old, it is the oldest known primate skeleton ever discovered, and harbors tantalizing clues about a pivotal branching point in humanity's evolutionary lineage.

Top image: CAS/Xijun Ni

Researchers have dubbed the newly identified primate Archicebus achilles, a name chosen to highlight the creature's interesting heel anatomy (which we'll get to in a moment), rather than its heroic proportions. In fact, Archicebus was actually quite small. Estimates put its weight at somewhere between 20 and 30 grams. Held in the palm of your hand, a specimen on the upper end of that range would have had roughly the same heft as $1.25 in quarters.

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That makes Archicebus one of the tiniest primates to ever live (smaller, even, than the pygmy mouse lemur – the world's tiniest living primate); but the combination of its diminutive size, its unprecedented age and its unique body plan make it one of the most fascinating evolutionary discoveries in recent memory.

"Archicebus marks the first time that we have a reasonably complete picture of a primate close to the divergence between tarsiers and anthropoids [the latter includes modern day monkeys, humans and apes]," said paleontologist Xijun Ni, lead author of the study that describes the specimen, which appears in this week's issue of Nature. "It represents a big step forward in our efforts to chart the course of the earliest phases of primate and human evolution."

Phylogeny diagram by M.A. Klingler via the Carnegie Museum of Natural History

Archicebus' strange remains were discovered in a rocky quarry in central China's Hubei Province. Today, the region's climate would best be described as subtropical. Fifty-five million years ago, however, the area was home to a lush tropical wilderness. Vast lake systems, shrouded in dense rainforest vegetation, dominated the landscape. But over millions of years, the region has wrung itself of much of its moisture, leaving behind layers upon layer of sedimentary rock, rich with fossilized remains. Like most fossils recovered from the region, Archebus' bones were discovered by splitting these layers of rock apart. This cleaved the skeleton into two complementary pieces, known as a "part" and "counterpart." By scanning the pieces separately and merging them into one, Ni's team was able to create a remarkably detailed 3D digital reconstruction –and what they found was unlike anything they'd ever seen.

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"Archicebus differs radically from any other primate, living or fossil, known to science," said study co-author Christopher Beard in a statement.

"It looks like an odd hybrid with the feet of a small monkey, the arms, legs and teeth of a very primitive primate, and a primitive skull bearing surprisingly small eyes. It will force us to rewrite how the anthropoid lineage evolved."

Image credit Mat Severson, Northern Illinois University

Study co-author Dan Gebo, and anthropologist and expert on the evolution of primate anatomy, says Archicebus' feet are particularly unusual. Its toes, for instance, are long and robust – a feature typical of primitive arboreal primates who needed their feet for grasping at tree branches. But its heel bone and metatarsals are surprisingly monkey-like – an "advanced features that you would not normally find in a primitive early Eocene fossil primate." Gebo elaborates:

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We have interpreted this new combination of features as evidence that this fossil is quite primitive and its unique anatomical combination is a link between the tarsier and monkey-ape branches of dry-nosed primates," he said. "This new view suggests that the advanced foot features of anthropoids (monkeys and apes) are in fact primitive for the entire lineage of dry-nosed primates.

The implications of Archicebus' anatomical mosaicism are manifold. At 55-million years old, this specimen is 7-million years older than any previously known fossil primate skeleton, and its small size upends earlier theories that Earth's earliest primates were large. Its location on the primate evolutionary tree provides one of the clearest looks yet at the divergence of monkeys, apes and humans from tarsiers. Meanwhile, its geographic origins could even challenge longstanding theories about the cradle of humanity.

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"In the past, many scientists believed that Africa was the continent of origin for all primates," said Gebo, "but it appears over the last decade that Asia is the more likely continent of origin, and this new skeleton supports that view."